1. Field of the Invention
The invention relates to a power control apparatus employed in an optical drive, and more particularly, to a power control apparatus and method by estimating the temperature rising effect of a laser diode to perform laser power control.
2. Description of the Related Art
Currently, a laser diode is employed as a light source in a conventional optical drive. In order to keep uniformity in crystallization throughout, the optical disks requires uniformity in laser power. However, as shown in FIG. 1, the optical power output of a laser diode varies greatly with the environmental temperature variations. That is, if the environmental temperature is rising, more current is required to keep the power level of a laser beam identical. If the power level is maintained at a specified value P, at different environmental temperatures, e.g., T3>T2>T1, different driving current levels, e.g., I3>I2>I1, need to be provided. Accordingly, an automatic power controller (APC) is required for the optical drive to stabilize the laser power output of the laser diode.
FIG. 2 is a block diagram illustrating a conventional optical drive. In order to correctly measure the optical power output of the laser diode in a conventional optical drive 200, a front photodiode 202 is used to detect the optical power level of a laser diode 201 and then generates an optical power current signal. Next, a current voltage converter 203 converts the optical power current signal into an optical power voltage signal, i.e., a front photodiode output (FPDO) signal. An encoder 206 receives recording data, encodes into three encoded signals EFM1, EFM2, EFM3, and then generates a sample signal SH. During a write mode, an erase mode and a bias mode, a sample and hold circuit 204 performs sample and hold operations to respectively generate measured voltage values VWS, VES, VBS corresponding to the write power level, the erase power level and the bias power level. During the three modes, an automatic power controller 207 respectively compares predetermined reference voltage values VWR, VER, VBR with the measured voltage values VWS, VES, VBS so as to correspondingly generate a write driving voltage VW, an erase driving voltage VE, a bias driving VB, therefore compensating the real laser power level. A laser power driver 205 receives the encoded signals EFM1, EFM2, EFM3, the write driving voltage VW, the erase driving voltage VE, and the bias driving VB and then converts them into currents signals for driving the laser diode 201. Consequently, the laser diode 201 emits laser beams towards a laser disk. Thus, during the above-mentioned three modes, the automatic power control is achieved in the optical drive 200 based on a closed-loop control strategy.
However, the essential perquisite for the closed-loop architecture is that the response speed of the front photodiode 202 must be fast enough to follow after the modulation speed of recording pulses of the laser diode 201; therefore, the optical power level of the laser diode 201 can be feedback correctly. Nevertheless, regarding to high speed or high density optical recording application combined with multi-pulse write waveforms, the response speed of the front photodiode 202 is usually slower than the modulation speed of recording pulses of the laser diode 201. This results in a failure to feedback the correct laser power level for automatic power control.
A first conventional solution to this problem is to equip the power feedback path with a peak envelope detection device. The peak envelope detection device continuously tracks the peaks and troughs of the FPDO signal using a peak/bottom hold method for power feedback control, as shown in FIG. 3. However, both the charge time constant and the discharge time constant of the peak envelope detection device needs to be calculated precisely; besides, while the response speed of the front photodiode 202 is much slower than the modulation speed of recording pulses of the laser diode 201, even the peak envelope detection device is not able to detect or reflect the real laser power level.
A second conventional solution to the above-mentioned problem is to use a method for automatic power control which is disclosed in the U.S. Patent Pub. No. 2005/0025018. A multi-pulse peak-hold device comprises a peak-hold circuit and a sample and hold circuit. FIG. 4 shows a relation of each signal inside the multi-pulse peak-hold device while a relatively low-speed front photodiode is being used. Referring to FIG. 4, the multi-pulse peak-hold device receives the FPDO signal to hold the maximum peak-hold output (MPHO) value of the FPDO signal. Next, the sample and hold circuit 204 samples the MPHO value of the FPDO signal according to the sampling signal SH. Following this, a reset signal is used to reinitialize the peak-hold circuit and therefore the MPHO value is cleared.
However, since the response speed of the front photodiode 202 is not fast enough, the MPHO value may be different from the real laser power level of the laser diode. Thus a power calibration procedure is required for calibrating a ratio of the measured power level to the real power level. The power calibration procedure comprises two steps as follows. Step 1: Measure the voltage level Y1 of the FPDO signal based on recording pulses with a modulation speed slow enough for the MPHO to reflect the real laser power level. Step 2: Measure the voltage level Y2 of the FPDO signal based on normal recording pulses. Finally, in operation, the ratio Y2/Y1 is used to compensate for the difference between the measured power level and the real laser power level.
A third conventional solution to the above-mentioned problem is to use a method for laser power control that is disclosed in the U.S. Patent Pub. No. 2005/0083828. The apparatus performs laser power control using an optional automatic power control area. Generally, due to the much higher channel bit rate, the Blue-ray encounters the above-mentioned problem frequently. Therefore, this method takes a Blue-ray disk as an example. The Blue-ray disk, rewritable format, version 1.0 defines physical data allocation and linking. Wherein, a data recording contains a sequence of recording unit blocks. There are a Run-in area and a Run-out area for each recording unit block. The optional automatic power control area is defined for laser power control in the Run-in area and the Run-out area.
FIG. 5 is a block diagram of another conventional optical drive. An optical drive 500 comprises an encoder/decoder controller 510, a sample and hold signal generator 530, a write strategy generator 520, three sample and hold circuits 204a, 204b, 204c, a CPU 550, an automatic power controller 207, a laser diode 201, a front photodiode 202, a current voltage converter 203 and a low-pass filter 540. According to the relative relationship between the recording speed and the bandwidth of the FPDO signal, the encoder/decoder controller 510 encodes a specific slower non-return to zero inverted (NRZI) signal pattern with a fixed duty ratio and then provides the NRZI signal to the laser power driver 205 for driving the laser diode 201. On the other hand, after receiving the FPDO signal, the low-pass filter 540 generates a FPDO average signal. For example, a fixed duty ratio of 50% would produce an average power level Pavg=Pw/2. The optical drive 500 also needs a power calibration procedure to obtain a relation of the laser power level to the FPDO signal, just the same as the second conventional solution to the above-mentioned problem. Different duty cycle ratios correspond to different calibration coefficients respectively.
All previously discussed solutions to the above-mentioned problem compensate the power feedback on condition that the response speed of the front photodiode 202 is slower than the modulation speed of recording pulses of the laser diode 201. In fact, as the power level is feedback in the closed-loop automatic power control mode, the result of performing the closed-loop automatic power control may be worse than that of performing an open-loop automatic power control if the compensation does not work well. In addition, each optical pickup head requires an off-line power calibration, so as to increase time and cost of optical-drive manufactures. Thus, the present invention discloses a compensation method different from the conventional solutions.